The present invention relates to a method for determining atmospheric air pressure in pressure-controlled fuel injection systems.
In vehicle engines having pressure-controlled fuel injection systems, i.e. systems in which the basic measured variable of the system is the intake manifold pressure, the exhaust back-pressure, which is dependent upon the prevailing atmospheric air pressure, influence the residual quantity of gas remaining in the engine cylinders during the exhaust cycle and hence the volume of fresh gas that can enter the cylinder on the next induction stroke. This has the result that, as the atmospheric air pressure decreases, the mixture in the cylinders of the engine becomes leaner. For example, when the engine is idling, the mixture becomes leaner, typically by approximately 4.5% per 1,000 m. altitude. In order for this error to be corrected, it is necessary to provide some means of determining the prevailing atmospheric air pressure.
A measure of the prevailing atmospheric air pressure can be obtained using the pressure value measured by the conventional intake manifold pressure sensor when the engine speed is still zero or extremely low, e.g. cranking speed. However, since the atmospheric pressure changes with the altitude at which the vehicle is operating, a measure of atmospheric pressure made initially by measuring the intake manifold pressure at start up of the engine requires correction to take account of a changing altitude of operation of the vehicle.